Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed to run at a quieter, more fuel efficient cruising speed. Typically, a transmission has a housing mounted to the vehicle structure, an input-shaft driven by an engine crankshaft through a clutch or torque converter, and an output-shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
An automatic transmission, also called an automatic gearbox, is a type of motor vehicle transmission that can automatically change gear ratios as the vehicle moves, freeing the driver from having to shift gears manually. Most automatic transmissions have a defined set of gear ranges, or step-ratios. Typically varying gear-elements of compound epicyclic planetary-gearsets are coupled together by varying shift-elements within the transmission to provide the step-ratios. The shift-elements may be actuated by hydraulic servos controlled by a valve body. Hydraulic fluid, or automatic transmission fluid, may be used to actuate and engage the shift-elements.